A hot electron or ballistic transistor typically includes emitter, base and collector zones, as in a junction transistor, but is characterized by an especially thin base through which change carriers pass at high velocity in short transit times without significant collisions.
A ballistic transistor of particular promise is one that has been described as a tunneling hot-electron transfer amplifier (THETA) device, and whose principles are described in a paper entitled, "Tunneling Hot-Electron Transfer Amplifier: A Hot Electron GaAs Device With Current Gain," published in Applied Physics Letters 47, (10) 15 November 1985, pp. 1105-1107 by M. Herklum et al. In this form of ballistic transistor, the applied voltage causes electrons to tunnel from emitter to base to provide an emitter current I.sub.e. The tunneling electrons are made to enter the base with excess kinetic energy larger than the effective collector barrier height and some of these electrons surmount the barrier and produce a collector current I.sub.c with a transfer coefficient A=I.sub.c /I.sub.e. Typically, to achieve the desired operation, the device has used heterojunctions between the emitter and the base and between the base and the collector, and is maintained at low temperatures to maximize the mean free path of the electrons.
Various materials have been proposed for use in ballistic transistors to achieve the desired operation. Generally, it has been recognized that there are desirable combinations of semiconductor materials with carriers of high mobility and long mean free paths that can be lattice matched to form a single crystal structure of high quality. In the past, emphasis has been on combinations involving Group III-Group V compounds including gallium, aluminum, indium, arsenic, etc as the constituent atoms, since such compounds were relatively well understood and believed to be of the most promise.
The Al.sub.1-x Ga.sub.x As/GaAs material system has been most used for devices so far. However, the relatively short mean free path of hot electrons in GaAs has let to some consideration being given to the InAs/GaInAsSb material system, which can also be lattice/matched. InSb has also been mentioned as a suitable material for a base material in a hot electron transistor. However, a severe problem in this case is that no other conventional Group III-Group V compound lattice-matches to it.
These alternatives to Al.sub.1-x Ga.sub.x As/GaAs have been regarded as potentially useful because moderately hot electrons in them would have relatively long mean free paths, allowing relatively thick base layers. This would decrease base sheet resistance, thus desirably decreasing the parasitic RC time constant of the device. It would be advantageous to dope a base region moderately heavily with electrons (greater than 5.times.10.sup.17 cm.sup.-3) in order to further reduce the base sheet resistance and hence the RC time constant, but this would greatly reduce the hot electron mean free path and also the electron mobility, because of increased electron-ionized donor and electron-electron scattering. The decreased electron mobility means that the base sheet resistance is not reduced as much as it otherwise would be. Moreover, the decreased hot electron mean free path means that thinner base layers muct be used to maintain an adequate transfer coefficient, further increasing the base sheet resistance and RC time constant, thus limiting the improvement in device switching time. Thus, it has been necessary to make comprises in device design and to utilize a lighter doping and higher sheet resistance for the base than is otherwise desirable.
These considerations have been largely described in a publication by A. F. J. Levi, J. R. Hayes, and R. Bhat in Applied Physics Letters, Volume 48, page 1609, in 1986. An additional problem that was described for these materials and for the HgTe/CdTe material system is that electrons that are too hot can lose energy by exciting electrons directly from the valence band of the base to the conduction band of the base. To avoid this loss mechanism, the hot electron energy, E, must be constrained as follows: EQU E&lt;E.sub.F +E.sub.G, ( 1)
where E.sub.F is the fermi energy relative to the conduction band edge and E.sub.G is the energy band gap. Thus, for narrow energy gap semiconductors such as InSb, inequality (1) becomes a significant constraint.